The present objectives of this research program concern three major topics: augmenting potentials and recruiting responses, the dorsal column nuclei, and the Bagley bundle and pyramidal tract. The cellular composition of the augmenting potentials and recruiting responses of the cat will be determined by extracellular recording of single neuron activity in pericruciate, somatosensory area I and association cortex. The intent is to discover whether these two phenomena reflect similar or dissimilar mechanisms, and to determine the relationship of each to the primary evoked response (and the association response). The net vertical current flows in depth and time during each of the above potentials will be measured on the assumption that I equals k delta V/delta x, whereas x is depth, and k does not vary significantly during these potentials. The intent is to discover the relationship between the patterns of single neuron activity and the associated current fields, with a view to enhancing our understanding of evoked potentials and ultimately the electroencephalogram. The internal organization of the dorsal column nuclei of the cat will be determined by extracellular recording of single neuron activity throughout the nucleus in response to cutaneous, cerebral and brain stem stimulation. The intent is to determine the overall circuitry within the nucleus, and to define in detail all routes by which the cerebral cortex can enhance and depress neuronal excitability within the nuclei. A neuronal population study will be made in the superior frontal gyrus of the goat and sheep, with special attention being given to the neurons that can be antidromically activated by stimulation of the bundle of Bagley and of the pyramidal tract. The intent is to determine the characteristics and extent of the excitatory receptive fields of these two classes of corticofugal neurons, and to discover their relationships to the various types of pyramidal tract neurons of the cat (and monkey). BIBLIOGRAPHIC REFERENCES: Harding, G. W., and A. L. Towe. An automated on-line laboratory for single neuron studies. Computers and Biomedical Research, 1976, 9:471-501. Mann, M. D., W. C. Holt, and A. L. Towe. Afferent modulation of the excitability of pyramidal tract fibers. Exp. Neurol., 1977, 55:414-435.